In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for forming an integrated circuit comprising such a fine pattern. However, the conventional photolithography method has been close to the resolution limit, while microsizing of semiconductor devices has been accelerated. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, is considered to be promising as an exposure technique for 45 nm or below. In this specification, “EUV light” means a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of from about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
EUV light is apt to be absorbed by any substances and the refractive indices of substances are close to 1 at this wavelength, whereby it is impossible to use a dioptric system like a conventional photolithography employing visible light or ultraviolet light. For this reason, for EUV light lithography, a catoptric system, i.e. a combination of a reflective photomask and a mirror, is employed.
A mask blank is a stacked member for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, formed thereon in this order (Patent Document 1).
In an EUV mask blank, the absorber layer had better be thin. In EUV lithography, the exposure light is not applied from the direction vertical to the EUV mask, but is applied from a direction inclined to the vertical direction by several degrees, usually 6°. If the absorber layer is thick, in EUV lithography, a shadow by the exposure light is cast on a mask pattern formed by removing a part of the absorber layer by patterning, whereby the shape accuracy and the dimensional accuracy of a mask pattern (hereinafter referred to as “transfer pattern”) to be transferred on a resist on a substrate such as a Si wafer by using the EUV mask will be deteriorated. This problem is more remarkable as the line width of a mask pattern formed on the EUV mask becomes smaller, and accordingly further reduction in the thickness of the absorber layer of the EUV mask blank has been expected.    Patent Document 1: JP-A-2004-6798 (U.S. Pat. No. 7,390,596)